Don't update gamePly after each move

We just need startup value to calculate available
thinking time. So remove from state.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/position.h

@@ -48,10 +48,10 @@ class Position;
 
 struct StateInfo {
   Key pawnKey, materialKey;
-  int castleRights, rule50, gamePly, pliesFromNull;
-  Square epSquare;
-  Score value;
   Value npMaterial[2];
+  int castleRights, rule50, pliesFromNull;
+  Score value;
+  Square epSquare;
 
   Key key;
   Bitboard checkersBB;
@@ -188,8 +188,8 @@ public:
   bool is_mate() const;
   template<bool SkipRepetition> bool is_draw() const;
 
-  // Number of plies from starting position
-  int game_ply() const;
+  // Plies from start position to the beginning of search
+  int startpos_ply_counter() const;
 
   // Other properties of the position
   bool opposite_colored_bishops() const;
@@ -255,6 +255,7 @@ private:
   Square castleRookSquare[16]; // [castleRight]
   StateInfo startState;
   int64_t nodes;
+  int startPosPly;
   Color sideToMove;
   int threadID;
   StateInfo* st;
@@ -421,8 +422,8 @@ inline bool Position::move_is_passed_pawn_push(Move m) const {
         && pawn_is_passed(c, move_to(m));
 }
 
-inline int Position::game_ply() const {
-  return st->gamePly;
+inline int Position::startpos_ply_counter() const {
+  return startPosPly;
 }
 
 inline bool Position::opposite_colored_bishops() const {